、。
When you come shopping, please do drop by.
Literal
Honorific-shopping [to-に] honorific-coming [of-の] occasion [at-には] by-all-means, honorific-drop-by please.
Highly polite keigo register, typical of shop advertising or service communication. お買い物 is the honorific form of 買い物 'shopping.' お越し is a honorific noun form of the verb 来る 'to come,' used only when speaking about the listener's action. ~の際 'on the occasion of, when' is a formal time marker. 是非 'by all means' emphasizes the sincerity of the invitation. お立ち寄りください is the お + verb-stem + ください honorific request formula — here applied to 立ち寄る 'to drop by.'